Coated optical fibers used in optical communications are generally made by coating optical fibers with a resin, whereas the resin usually constitutes two layers consisting of a primary coating layer as the inner layer and a secondary coating layer as the outer layer.
An example of such coated optical fibers is disclosed in Japanese Patent Application Laid-Open No. HEI 8-248250. The coated optical fiber disclosed in this publication uses a primary coating layer having a Young's modulus of 1.0 to 3.0 MPa and a glass transition point of −10° C. or lower, and a secondary coating layer having a Young's modulus of at least 400 MPa.